Spider silks are nature's high-performance polymers, obtaining extraordinary toughness due to a combination of strength and elasticity. Up to seven specialized glands exist in spiders, which produce a variety of silk fiber types with different mechanical properties and functions. Dragline silk, produced by the major ampullate gland, is the toughest fiber, and on a weight basis it outperforms man-made materials, such as high tensile steel and Kevlar. The properties of dragline silk are attractive in development of new materials for medical or technical purposes.
Dragline silk consists of two main polypeptides, mostly referred to as major ampullate spidroin (MaSp) 1 and 2, but to ADF-3 and ADF-4 in Araneus diadematus. These proteins have apparent molecular masses in the range of 200-720 kDa, depending on sample age and conditions of analysis, but no full-length dragline spider silk gene has yet been reported. The properties of dragline silk polypeptides are discussed in Huemmerich, D. et al. Novel assembly properties of recombinant spider dragline silk proteins. Curr. Biol. 14, 2070-2074 (2004). The known dragline silk spidroins are composed of highly iterated blocks of alternating alanine-rich segments, forming crystalline β-sheets in the fiber, and glycine-rich segments which are more flexible and mainly lack ordered structure. The C-terminal region is non-repetitive, highly conserved between species, and adopts α-helical conformation. The N-terminal region of dragline silk proteins has not been characterized until very recently, revealing an N-terminal domain that is highly conserved between different spidroins, and also between different spider species (Rising, A. et al. N-terminal nonrepetitive domain common to dragline, flagelliform, and cylindriform spider silk proteins. Biomacromolecules 7, 3120-3124 (2006)).
The mechanical properties of dragline silk varies between species; Euprosthenops sp dragline silk is stiffer, stronger (requires more force to break) and less extendible than dragline silk from e.g. Araneus diadematus or Nephila clavipes. Dragline silk from Euprosthenops sp appears to have a greater proportion of crystalline β-sheet structure than dragline silk from Araneus diadematus, most likely due to that the Euprosthenops sp MaSp has the highest polyalanine content among all species analyzed so far (Pouchkina-Stantcheva, N. N. & McQueen-Mason, S. J. Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)).
Attempts to produce artificial spider silks have employed natural or synthetic gene fragments encoding dragline silk proteins, since no full-length gene has yet been reported. Recombinant dragline silk proteins have been expressed in various systems including bacteria, yeast, mammalian cells, plants, insect cells, transgenic silkworms and transgenic goats. See e.g. Lewis, R. V. et al. Expression and purification of a spider silk protein: a new strategy for producing repetitive proteins. Protein Expr. Purif. 7, 400-406 (1996); Fahnestock, S. R. & Irwin, S. L. Synthetic spider dragline silk proteins and their production in Escherichia coli. Appl. Microbiol. Biotechnol. 47, 23-32 (1997); Arcidiacono, S. et al. Purification and characterization of recombinant spider silk expressed in Escherichia coli. Appl. Microbiol. Biotechnol. 49, 31-38 (1998); Fahnestock, S. R. & Bedzyk, L. A. Production of synthetic spider dragline silk protein in Pichia pastoris. Appl. Microbiol. Biotechnol. 47, 33-39 (1997); and Lazaris, A. et al. Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science 295, 472-476 (2002).
WO 2004/016651 (The University of York) discloses nucleic acid sequences coding for internal, repetitive parts of MaSp1 proteins from Euprosthenops sp. No protein is expressed.
Huemmerich, D. et al. Primary structure elements of spider dragline silks and their contribution to protein solubility. Biochemistry 43, 13604-13612 (2004) discloses a synthetic gene, “(AQ)12NR3”, coding for repetitive Ala-rich and Gly/Gln-rich fragments and a non-repetitive fragment, all derived from ADF3 from Araneus. The gene is expressed into a soluble protein (59.8 kD, >528 aa), which aggregates but does not form polymers or fibers. The alanine content of the protein is 10-15%.
WO 03/057727 discloses expression of soluble recombinant silk polypeptides in mammalian cell lines and animals. One expressed silk polypeptide (ADF-3; 60 kD, 652 aa) consists of a repetitive unit and a non-repetitive hydrophilic domain. Another expressed silk polypeptide (ADF-3 His; 63 kD, 677 aa) consists of a repetitive unit, a non-repetitive hydrophilic domain, a c-myc epitope and a six-Histidine tag. The repetitive unit has a low content of Ala (10-20%). The obtained silk polypeptides exhibit poor solubility in aqueous media and/or form precipitates. Since the obtained silk polypeptides do not polymerize spontaneously, spinning is required to obtain polymers or fibers.
Several factors complicate the expression of dragline silk proteins. Due to the highly repetitive nature of the genes, and the concomitant restricted amino acid composition of the proteins, transcription and translation errors occur. Depletion of tRNA-pools in microbial expression systems, with subsequent discontinuous translation, leading to premature termination of protein synthesis might be another reason. Other reasons discussed for truncation of protein synthesis are secondary structure formation of the mRNA, and recombination of the genes. Native MaSp genes larger than 2.5 kb have been shown to be instable in bacterial hosts. Additionally, there are difficulties in maintaining the recombinant silk proteins in soluble form, since both natural-derived dragline silk fragments and designed block copolymers, especially MaSp1/ADF-4-derived proteins, easily self-assemble into amorphous aggregates, causing precipitation and loss of protein. See Huemmerich, D. et al. Primary structure elements of spider dragline silks and their contribution to protein solubility. Biochemistry 43, 13604-13612 (2004) and Lazaris, A. et al. Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science 295, 472-476 (2002).